The subject matter disclosed herein relates generally to sensors, and more particularly, to embodiments of a sensing device that are configured to measure properties of caustic working fluids such as fuel, coolant, oils, and hydraulic fluids used in an automotive vehicle.
Sensor assemblies can comprise threaded metal or plastic that can form discrete threaded housings or threaded interfaces, which can be incorporated into other functional components such as fluid fittings. These sensor assemblies may incorporate sensing elements that are responsive to one or more properties of a working fluid (e.g., fuel). Temperature sensors, pressure sensors, flow sensors, and the like are all suitable sensing elements that can be incorporated as part of the sensor assembly. Certain applications may require that these sensor assemblies, as well as or in addition to the fluid fittings in which the sensor assembly is incorporated, are constructed so that each can withstand the physical and the chemical characteristics particular to the working fluid and/or the environment that utilizes the working fluid. Exemplary environments can include systems such as fuel, coolant, lubrication, hydraulic, and brake systems, all of which can be found in automobiles.
While some sensors are compatible with fittings for use in these environments, such as sensors that can monitor properties of the working fluids in automotive systems, few of these sensors incorporate semiconductor devices such as semiconductor-based die, ceramic-based die, or other die with similar capacitive properties. One reason for this is the inadequate construction of the sensor. For example, circuitry for many silicon-based die (e.g., piezo-resistive pressure sensor die) are manufactured on silicon wafers. These wafers may require a supportive structure that is bonded to the backside of the wafer. This structure can be constructed of materials (e.g., glass) that have a coefficient of thermal expansion (“CTE”) that is similar to the CTE of the silicon wafer. Further processing of the wafer can result in generally cubic sensing packages that comprise the silicon/glass assembly. To form the sensor, these cubic sensing packages can be attached to a secondary substrate such as polymeric thermoplastics, which are generally selected because these materials are resistant to the chemical properties of the working fluid.
An epoxy is typically used to bond the glass portion of the wafer/glass assembly to the substrate. However, epoxies tend to act on the surface microstructure as between the glass and the plastic substrate. This action forms a mechanical bond, which can degrade when exposed to the working fluid. For example, the properties of the mechanical bond can change over time as the hydrocarbons in the epoxy cross-link and change their material characteristics in response to temperature and chemical attack. Moreover, because the epoxy materials that are used to bond the glass and plastic together have a CTE from about 20 ppm/° C. to about 100 ppm/° C., these epoxies expand and contract at a rate that is greater than either the glass or silicon of the sensing package. This rate can cause cyclic shear fatigue, thus making epoxies poor bonding materials for environments that exhibit large deviations in temperatures and/or high pressures.
There are fittings that are constructed to overcome some of these issues. Such sensors may incorporate ceramic capacitive circuits that are printed on stainless steel foil. Fittings that utilize this configuration, however, often comprise large stainless steel housings and connective mechanisms (e.g., threaded connectors, brazed and welded joints) for securing the housing to the fluid-carrying pipe. This construction causes the resulting fitting to be large and bulky, characteristics which are problematic and ill-suited for many automotive systems. For example, space constraints in the automobile structure can limit the packaging size so that large fittings may necessitate costly design changes to the components and their layout within the vehicle structure.
Therefore, it would be advantageous to provide a sensor that can withstand caustic working fluids, but that is designed and manufactured for robust and varied applications. It would be advantageous, for example, to provide a sensor for measuring properties of caustic working fluids with improved accuracy and reliability, but that is constructed in a manner compatible with fittings that meet cost, size, and other design constraints of the automotive industry.